Wireless power transfer systems with integrated impedance matching and methods for using the same

ABSTRACT

A conventional wireless power transfer (WPT) system based on resonant inductive coupling typically operates at a peak power efficiency at the expense of a energy transfer rate. Impedance matching circuits can increase the energy transfer rate, but tend to increase the complexity, form factor, and weight of the WPT system. To overcome these limitations, a WPT system is described herein that includes a resonant circuit with integrated impedance matching. The resonant circuit includes a first coil, a first capacitor in series with the first coil, a second coil in series with the first coil and the first capacitor, and a second capacitor in parallel with the first coil and the first capacitor. The inductor coils and capacitances are tailored to increase the voltage gain and, thus, the energy transfer rate. The inductor coils also transmit or receive power, thus increasing the energy transfer rate and the power efficiency.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/964,251, filed on Jul. 23, 2020, and entitled “Wireless PowerTransfer Systems Having Concentric Coils,” which is a national-stageapplication, under 35 U.S.C. § 371, of International Application No.PCT/US2019/015352, filed on Jan. 28, 2019, and entitled “Wireless PowerTransfer Systems with Integrated Impedance Matching and Methods forUsing the Same,” which in turn claims priority, under 35 U.S.C. §119(e), to U.S. Application No. 62/622,288, filed on Jan. 26, 2018, andentitled “Wireless Power Transfer System (WPTS) with Dual Coils,” eachof which is incorporated herein by reference in its entirety.

BACKGROUND

A wireless power transfer (WPT) system transfers power from atransmitter to a receiver based on the emission and absorption ofelectromagnetic waves. The transmitter and the receiver in a WPT systemdo not need to be physically connected, thus eliminating the need forwiring when charging and/or powering electronic devices. WPT systemshave utility for a broad range of applications including, but notlimited to, charging batteries in electric vehicles, trains, or buses;powering home appliances, such as lighting, televisions, and climatecontrol devices; charging autonomous robots or drones; and poweringbiomedical devices, especially devices implanted in the human body.

Over the years, various types of WPT systems have been developed, whichare differentiated, in part, by how the transmitter is coupled to thereceiver. For instance, WPT systems have been developed based oninductive coupling, capacitive coupling, and magnetodynamic coupling,which have different advantages and disadvantages in terms of theoperating frequency, low power/high power compatibility, and operatingdistance. WPT systems based on inductive coupling are among the mostwidely used in commercial applications, especially for applicationswhere higher energy transfer rates (also referred to as “power level”)are preferable (e.g., greater than 1 kW).

SUMMARY

A conventional WPT system based on resonant inductive coupling typicallyexhibits a peak energy transfer rate and a peak power efficiency atdifferent operating frequencies. This is called the frequency splittingeffect. As a result of the frequency splitting effect, a conventionalWPT system cannot operate at both the peak energy transfer rate and thepeak power efficiency simultaneously. In some conventional WPT systems,impedance matching circuits are used to partially compensate a reductionin the energy transfer rate caused by the frequency splitting effectwhen operating at the resonant frequency. However, conventional matchingcircuits typically include large, bulky circuit components (e.g.inductors, capacitors) that increase the complexity, size, and weight ofthe WPT system.

The present disclosure is thus directed to a wireless power transfer(WPT) system that utilizes a resonant circuit with integrated impedancematching. The resonant circuit includes inductor coils and capacitorsconfigured to compensate for the frequency splitting effect byincreasing the voltage gain of the transmitter and/or the receiver, thusincreasing the energy transfer rate. By integrating the voltage boostingfunctionality into the resonant circuit, the high voltage components inthe WPT system are confined to the circuitry used to transmit orreceiver energy. This substantially reduces the voltage (e.g., from 400V in conventional electric vehicles (EVs) to less than 67 V) used inother electrical components coupled to the WPT system, such as atransmitter driving circuit, a battery, and any wiring used forelectrical connections, thus improving the overall safety of the WPTsystem.

The inductor coils are also used to wirelessly exchange energy betweenthe transmitter and the receiver, increasing both the energy transferrate and the power efficiency. In this manner, the resonant circuitenables the WPT system to operate at a peak power efficiency and ahigher energy transfer rate than conventional systems. Furthermore, theintegrated functionality provided by the resonant circuit reduces thenumber of discrete components used in the WPT system, enabling a smallerform factor and lighter weight system. Methods for using the WPT systemare also described in the present disclosure.

In one exemplary design, a wireless power receiver includes a resonantcircuit. The resonant circuit includes a first coil and a firstcapacitor coupled in series to the first coil. The resonant circuit alsoincludes a second coil, coupled in series to the first coil and thefirst capacitor, and a second capacitor, coupled in parallel to thefirst coil and the first capacitor. The first coil and the second coilmay both provide impedance matching to increase the voltage gain of theresonant circuit at a desired operating frequency. The first coil andthe second coil are also configured to receive energy from one or morecoils in a transmitter. The wireless power receiver is also electricallycoupled to a load that receives power from the transmitter.

An exemplary wireless power transmitter may have a substantially similardesign to the wireless power receiver described above with the primarydifference being the replacement of the load with a power source. Thetransmitter may also include a resonant circuit with inductor coils andcapacitors configured to increase the voltage gain of the transmitter.The inductor coils in the transmitter may also transmit energy to thefirst coil/second coil in the receiver.

A WPT system may thus include one or both of the wireless power receiverand the wireless power transmitter. In one exemplary design, theinductor coils in both the transmitter and the receiver may bemagnetically coupled, thus increasing the energy transfer rate and thepower efficiency. Additionally, the resonant frequency of thetransmitter and/or the receiver may be tuned to match the operatingfrequency of the WPT system in order to increase the power efficiency.

An exemplary wireless power receiver may include a resonant circuit toreceive power from a wireless power transmitter via wireless magneticresonance charging at a voltage gain of about 1 and an efficiency of atleast 95% at a resonant frequency between about 80 kHz and about 90 kHz.The resonant circuit may include a first coil, a second coil coupled inseries with the first coil, the first coil and the second coil configureto receive the power, a first capacitor coupled in series with the firstcoil, and a second capacitor coupled in parallel with the first coil andthe first capacitor. The first coil and the second coil may beconfigured to receive energy from a third coil and a fourth coil in thewireless power transmitter. The first coil may have an inductance ofabout 0.1 μH to about 100 μH, the first capacitor may a capacitance ofabout 0.01 μF to about 100 μF, the second coil may have an inductance ofabout 0.1 μH to about 100 μH, and the second capacitor may have acapacitance of about 0.01 μF to about 100 μF. In some designs, the firstcoil may have an inductance of about 1 μH to about 20 μH, the firstcapacitor may have a capacitance of about 0.05 μF to about 2 μF, thesecond coil may have an inductance of about 1 μH to about 20 μH, and thesecond capacitor may have a capacitance of about 0.05 μF to about 2 μF.

An exemplary method of transferring power wirelessly may includereceiving the power, from at least one coil, with a first coil and asecond coil in series with the first coil at a resonant frequencybetween about 80 kHz and about 90 kHz, a voltage gain of about 1.0, andan efficiency of at least 95%. The method may further includepositioning the first coil within about 50 mm of the at least one coil.The at least one coil in the method may include a third coil matchingthe first coil and a fourth coil in series with the third coil andmatching the second coil. The method may further include the step ofrunning a first current through the third coil and the fourth coil wherethe first current induces a second current running through the firstcoil and the second coil. The phase difference between the first currentat the third coil and the first current at the fourth coil may be lessthan about 20 degrees or less than about 10 degrees.

Another exemplary wireless power receiver may include a first coil, asecond coil in series with the first coil, a first capacitor in serieswith the first coil and the second coil, and a second capacitor inparallel with the first coil and the first capacitor. The resonantfrequency of the wireless power receiver may be about 87 kHz. Thewireless power receiver may have a voltage gain of about 1 over a bandfrom about 80 kHz to about 90 kHz. The wireless power receiver may havea power transfer efficiency of at least 95% over a band from about 80kHz to about 90 kHz. The first coil may be concentric with the secondcoil. The first coil may be co-planar with the second coil. The firstcoil may be stacked on the second coil. The first coil may be a firstflat spiral coil and the second coil may be a second flat spiral coil.The first coil and the second coil may each have an outer diameter equalto or less than 220 mm. The second coil may be connected in seriesbetween the first coil and the first capacitor. The first capacitor mybe connected series between the first coil and the second coil. Thefirst coil may have an inductance of about 0.1 μH to about 100 μH, thefirst capacitor may a capacitance of about 0.01 μF to about 100 μF, thesecond coil may have an inductance of about 0.1 μH to about 100 μH, andthe second capacitor may have a capacitance of about 0.01 μF to about100 μF. In some designs, the first coil may have an inductance of about1 μH to about 20 μH, the first capacitor may have a capacitance of about0.05 μF to about 2 μF, the second coil may have an inductance of about 1μH to about 20 μH, and the second capacitor may have a capacitance ofabout 0.05 μF to about 2 μF.

It should be appreciated that all combinations of the foregoing conceptsand additional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein. It should also be appreciated that terminologyexplicitly employed herein that also may appear in any disclosureincorporated by reference should be accorded a meaning most consistentwith the particular concepts disclosed herein.

BRIEF DESCRIPTION OF DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1A shows a circuit diagram of a conventional wireless powertransfer (WPT) system.

FIG. 1B shows a chart of the voltage gain versus frequency for theconventional WPT system of FIG. 1A.

FIG. 1C show a chart of the power efficiency versus frequency for theconventional WPT system of FIG. 1A.

FIG. 2A shows a circuit model of a conventional WPT system with a πmatching network.

FIG. 2B shows a circuit model of a conventional WPT system with an LCmatching network.

FIG. 3A shows a circuit model of an exemplary WPT system with two energycoupling and matching coils in the transmitter and two energy couplingand matching in the receiver.

FIG. 3B shows a circuit model of another exemplary WPT system where thelocation of the resonant capacitor and the first coil in the resonantcircuit is switched. This circuit model may also use a tapped inductorcoil.

FIG. 3C shows a circuit model of another exemplary WPT system using atapped inductor coil based on the circuit model of FIG. 3B.

FIG. 4A shows an exemplary first coil and second coil as flat spiralcoils where the second coil is disposed in the center of and concentricwith the first coil.

FIG. 4B shows an exemplary first coil and second coil as flat spiralcoils where the first coil is disposed in the center of and concentricwith the second coil.

FIG. 4C shows an exemplary first coil and second coil as flat spiralcoils where the second coil is disposed on and concentric with the firstcoil.

FIG. 5 shows an exemplary WPT system with a transmitter and a receivercomprised of flat spiral coils.

FIG. 6A shows a chart of the voltage gain versus frequency of theexemplary WPT system shown in FIG. 5 .

FIG. 6B shows a chart of the power efficiency versus frequency of theexemplary WPT system shown in FIG. 5 .

FIG. 7A shows a chart of the power efficiency as a function coilmisalignment along the radial axis, X, for the exemplary WPT systemshown in FIG. 5 .

FIG. 7B shows a chart of the power efficiency as a function of coilseparation along the axial axis, Z, for exemplary WPT system shown inFIG. 5 .

DETAILED DESCRIPTION

A conventional WPT system based on inductive coupling typically includesan inductor coil in the transmitter and the receiver, respectively,which exchange power via magnetic field coupling. In order to increasethe energy transfer rate, power efficiency, and range of operation, thetransmitter and the receiver in the WPT system are often configured tooperate at resonance.

FIG. 1A shows a conventional WPT system 100 with resonant inductivecoupling. The WPT system 100 includes a transmitter 110 and a receiver120. The transmitter 110 includes an energy coupling coil 111 and aresonant capacitor 112 (CO coupled electrically in series to a powersource 115 (V_(i)). The receiver 120 similarly includes an energycoupling coil 121 and a resonant capacitor 122 (C_(r)) coupledelectrically in series to a load 125. The energy coupling coils 111(121) are represented by the winding resistance R_(t) (R_(r)) and theinductance L_(t) (L_(r)).

The power source 115 supplies a voltage, which drives an electriccurrent through the energy coupling coil 111 thus generating a magneticfield. The magnetic field may then be absorbed, at least in part, by theenergy coupling coil 121 creating an electric current that then flowsthrough the load 125. In this manner, electric power is transferredwirelessly between the transmitter 110 and the receiver 120. Theresonant capacitors 112 and 122 are typically tailored to substantiallyreduce the input and output impedance, thus increasing the energytransfer rate between the transmitter 110 and the receiver 120.

The power efficiency of a typical WPT system may be defined as follows,

η=P _(R) /P _(T)  (1)

where η is the power efficiency, P_(R) is the total energy received bythe energy coupling coil 121 in the receiver 120 over a time intervalΔt, and P_(T) is the total energy transmitted by the energy couplingcoil 111 in the transmitter 110 over the time interval Δt. The energytransfer rate may be defined as the power, or the rate of energytransfer per unit time, received by the energy coupling coil 121 in thereceiver 120. The magnitude of the energy transfer rate thus depends, inpart, on the power transmitted by the energy coupling coil 111. Theenergy transfer rate is proportional to the voltage gain (defined as theratio of the output voltage V_(o) at the load 125 and the input voltageV_(i) at the power source 115) between the transmitter 110 and thereceiver 120. Thus, an increase in the energy transfer rate iscorrelated to an increase in the voltage gain.

The coupling between the energy coupling coils 111 and 121 may becharacterized by a magnetic coupling coefficient, k. Generally, k mayincrease by increasing the diameter of the energy coupling coils 111 and121 and/or reducing the gap or distance between the energy couplingcoils 111 and 121. When the magnetic coupling coefficient k increasesand/or the gap between the energy coupling coils 111 and 121 decreases,the power efficiency typically increases. Thus, the energy coupling coil121 can receive more of the energy transmitted by the energy couplingcoil 111 due to the stronger coupling. However, under over-coupledconditions, the energy transfer rate typically decreases, whichindicates a reduction in the amount of energy transmitted by the energycoupling coil 111.

The induced voltage at the energy coupling coil 111 in the transmitter110, caused by the energy coupling coil 121 in the receiver 120,increases as the coupling between the transmitter 110 and the receiver120 increases. This induced voltage has a polarity opposite to theapplied voltage from the power source 115, thus impeding the inputcurrent flowing through the transmitter 110 resulting in a reduction inthe voltage gain and the energy transfer rate. This undesirable voltageis proportional to the electric current flowing through the energycoupling coil 121 and the magnetic coupling coefficient k and inverselyproportional to the gap size. For example, when the WPT system 100operates at the resonant frequency and the gap between the energycoupling coils 111 and 121 decreases, the electric current in the energycoupling coil 121 of the receiver 120 increases. This, in turn, inducesa larger voltage at the energy coupling coil 111, thus reducing thevoltage gain and the energy transfer rate. In other words, making gapbetween the coils increases power transfer efficiency and decreases theenergy transfer rate.

Referring to the WPT system 100 shown in FIG. 1A, the WPT system 100 maybe configured to support a resonant frequency in the series LC circuitsformed by the energy coupling coil 111 and resonant capacitor 112 (andthe energy coupling coil 121 and the resonant capacitor 122), which maybe tuned to the operating frequency of the WPT system 100. As shown inFIG. 1A, E_(t) represents the voltage at the transmitting coil inducedby the magnetic field generated by the receiving coil current. E_(r)represents the voltage at the receiving coil induced by the magneticfield generated by the transmitting coil current. As described above,stronger coupling (e.g., a higher magnetic coupling coefficient k) mayincrease the power efficiency of the WPT system 100. However, strongercoupling also leads to a stronger frequency splitting effect.

FIGS. 1B and 1C show the voltage gain and the power efficiency of theWPT system 100, respectively, at various values of the magnetic couplingcoefficient k. In FIG. 1B, the voltage gain exhibits two peaks, whichindicates the resonant mode splits into an even and odd mode. As shown,the even/odd modes shift to smaller/larger frequencies relative to theresonant frequency, f_(r), as the coupling coefficient k increases,which leads to a reduction in the voltage gain at the resonantfrequency. In FIG. 1C, the power efficiency exhibits a peak at theresonant frequency, which increases as the coupling coefficientincreases. The power efficiency tends to decrease at the off-resonantfrequencies corresponding to the even and odd modes. As a result, thepower efficiency and the energy transfer rate, as indicted by thevoltage gain, exhibit peak values that occur at different frequencies.Thus, the conventional WPT system 100 cannot operate at both the peakenergy transfer rate and the peak power efficiency simultaneously due tothis frequency splitting effect.

Several approaches have been developed in an effort to overcome thefrequency splitting effect described above in the WPT system 100. In oneexample, the energy coupling coils 111 and 121 are intentionallyconfigured to have different electrical properties in order to reducethe magnetic coupling coefficient k, thus reducing the induced voltagein the energy coupling coil 111. Specifically, the diameter of theenergy coupling coil 121 was chosen to be substantially smaller orlarger than the diameter of the energy coupling coil 111 such that theratio of the diameters deviates substantially from unity. However,reducing the magnetic coupling coefficient k in this manner reduces theenergy transfer rate and the power efficiency of the WPT system.

In another example, an active matching tuning circuit is integrated intothe transmitter 110. The active matching tuning circuit adjusts theresonant frequency of the transmitter 110 to match the operatingfrequency of the WPT system 100, thus enabling the WPT system 100 tooperate at the peak power efficiency. However, the voltage gain and thusthe energy transfer rate are sacrificed in this design due to thefrequency splitting effect. Additionally, switches and capacitors ratedfor high-voltage and high-current operation are used in the resonantcircuit, which can substantially increase the cost, size, weight,complexity, and power loss of the WPT system 100. Thus, this approach isnot desirable, especially for high power applications such as electricvehicle (EV) battery charging.

In yet another example, a frequency tracking method is used where theoperating frequency of the WPT system 100 is instead adjusted to matchthe resonant frequency in order to match the peak power efficiency. Thisapproach does not include the use of variable capacitors or arrays ofhigh-power rating tuning capacitors and switches, thus greatlysimplifying the WPT system 100 and reducing cost. However, the voltagegain and thus the energy transfer rate are again sacrificed in thisdesign due to the frequency splitting effect. Furthermore, this approachmay not be feasible in applications where the operating frequency isconstrained to a particular range that does not include the resonantfrequency. For example, the WPT system 100 for an EV battery chargingmay operate at a frequency ranging between 81.38 kHz and 90 kHz with aresonant frequency less than 81.38 kHz or greater than 90 kHz.

In yet another example, impedance matching circuits may be integratedinto the transmitter 110 and/or the receiver 120 in order to increaseboth the energy transfer rate and the power efficiency. FIG. 2A shows aWPT system 200 a first π configuration impedance matching circuit 217 inthe transmitter 210 and a second π configuration impedance matchingcircuit 227 in the transmitter 220. FIG. 2B shows a WPT system 202 afirst LC configuration impedance matching circuit 219 in the transmitter213 and a second LC configuration impedance matching circuit 229 in thetransmitter 223.

Each matching circuit 217, 219 (227, 229) includes an inductor coil inseries with an energy coupling coil 211 (221) and a resonant capacitor212 (222). Each matching circuit 217, 219 (227, 229) may also includeone or more capacitors, depending on the circuit configuration (e.g.,the π configuration, LC configuration), in parallel with the energycoupling coil 211 (221) and the resonant capacitor 212 (222). A powersource 215 is coupled in series in the transmitter 210, 213 and a load225 is coupled in series in the receiver 220, 223. The inductors andcapacitors in the impedance matching circuits 217 (219) and 227 (229)increase the voltage gain at the resonant frequency to compensate forthe frequency splitting effect. Thus, the WPT systems 200 and 202 mayoperate at the resonant frequency to match the peak power efficiencywithout substantial reductions to the voltage gain and the energytransfer rate.

However, the impedance matching circuits 217, 219, 227, and 229 in theseWPT systems 200 and 202 are typically discrete electrical componentscoupled to the energy coupling coils 211 and 221 and the resonantcapacitors 212 and 222 with separate wiring. It is also common for theimpedance matching circuits 217, 219, 227, and 229 to include large,heavy inductor coils. Thus, these impedance matching circuits 217, 219,227, and 229 tend to increase the size, the complexity, and the weightof the WPT systems 200 and 202.

A WPT System with a Resonant Circuit Having Integrated ImpedanceMatching

Using an integrated impedance matching circuit in a WPT system increasesboth the energy transfer rate and the power efficiency without undulyincreasing the WPT system's size or form factor. The circuit includesinductor coils and capacitors that increase the voltage gain, thuscompensating the reductions in the energy transfer rate caused by thefrequency splitting effect described above. The inductor coils used forimpedance matching may also be positioned proximate to one another orintegrated as a single inductor coil (e.g., a tapped inductor coil) inorder to transmit and receive power in the transmitter and the receiver,respectively. By integrating the impedance matching functionality in theresonant circuit, the impedance matching circuits used in conventionalWPT systems may be eliminated, substantially reducing the number ofdiscrete electronic components used and, hence, the form factor andweight of the WPT system.

The WPT system described herein may have several benefits. For example,the WPT system may have a smaller operating area, thus reducingpotential safety hazards associated with undesirable heating of objectsthat may be inadvertently disposed between the transmitter and thereceiver. A smaller, lighter weight WPT system may also enableinstallation onto systems with stringent space and weight requirements,such as the side panels or bumpers of an EV, a charging platform for adrone or robot, or in confined spaces in vehicle trailers. In oneexemplary application, one or more WPT systems may be installed on theundercarriage, side panels, and/or front/rear bumpers of an EV. TheseWPT systems may be used to charge the batteries of the EV when the EV isparked in a garage or parking lot or when the EV is stationary at atraffic light. Additionally, the WPT systems may also be used totransfer power between two vehicles on the road.

FIG. 3A shows an exemplary design for a WPT system 1000. The WPT system1000 may include a transmitter 1100 and a receiver 1200. The transmitter1100 may include a resonant circuit 1102 with a first coil 1110, a firstcapacitor 1120 (C_(b)) coupled in series to the first coil 1110, asecond coil 1130 coupled in series to the first coil 1110 and the firstcapacitor 1120, and a second capacitor 1140 (C_(a)) coupled in parallelto the first coil 1110 and the first capacitor 1120. The resonantcircuit 1102 may be electrically coupled to a power source 1150 thatsupplies energy in the WPT system 1000. The first coil 1110 may berepresented by a winding resistance (R_(b)) and a self-inductance(L_(b)). The second coil 1130 may be represented by a winding resistance(R_(a)) and a self-inductance (L_(a)). R_(a) and L_(a) may include theresistance and the inductance, respectively, of the wires connecting thepower source 1150 to the second coil 1130.

Similarly, the receiver 1200 may include a resonant circuit 1202 with athird coil 1210, a third capacitor 1220 (C_(d)) coupled in series to thethird coil 1210, a fourth coil 1230 coupled in series to the third coil1210 and the third capacitor 1220, and a fourth capacitor 1240 (C_(c))coupled in parallel to the third coil 1210 and the third capacitor 1220.The resonant circuit 1202 may be electrically coupled to a load 1250that receives energy in the WPT system 1000. The third coil 1210 may berepresented by a winding resistance (R_(d)) and a self-inductance(L_(d)). The fourth coil 1230 may be represented by a winding resistance(R_(e)) and a self-inductance (L_(c)). R_(c) and L_(c) may include theresistance and the inductance, respectively, of the wires connecting theload 1250 to the fourth coil 1230.

Codependency of Electronic Properties in the WPT System

During power transfer, the inductors in the WPT system 1000 may all bemagnetically coupled to one another, giving rise to induced voltages ateach inductor coil with contributions from the other inductor coils inthe system. As shown in FIG. 3A, the voltage drop across the first coil1110 also includes the induced voltages that arise at the first coil1110 due to current flowing through the second coil 1130 (E_(ba)), thethird coil 1210 (E_(bd)), and the fourth coil 1230 (E_(bc)). Similarly,the voltage drop across the second coil 1130 includes induced voltagesfrom the first coil 1110 (E_(ab)), the third coil 1210 (E_(ad)), and thefourth coil 1230(E_(ac)). The voltage drop across the third coil 1210includes induced voltages from the first coil 1110 (E_(db)), the secondcoil 1130 (E_(da)), and the fourth coil 1230 (E_(dc)). Finally, thevoltage drop across the fourth coil 1230 includes induced voltages fromthe first coil 1110 (E_(cb)), the second coil 1130 (E_(ca)), and thethird coil 1210 (E_(cd)).

Because the inductor coils in the WPT system 1000 are magneticallycoupled to one another, the electronic and structural properties of theinductor coils and the capacitors are codependent and should be tunedcollectively to increase both the energy transfer rate and the powerefficiency of the WPT system 1000. This is in stark contrast toconventional WPT systems with impedance matching circuits where theelectronic properties of the matching circuit in a transmitter or areceiver are typically tuned independently of the other electroniccomponents in the transmitter and the receiver.

For example, the first coil 1110 and the second coil 1130 in thetransmitter 1100 should be tuned such that the phase difference betweenthe electric currents flowing through the first coil 1110 and the secondcoil 1130 is small. A smaller phase difference corresponds to lessdestructive interference between the magnetic fields generated by thefirst coil 1110 and the second coil 1130 and thus greater powertransfer. For the WPT system 1000, the phase difference between theelectric currents flowing through the first coil 1110 and the secondcoil 1130 (or the third coil 1210 and the fourth coil 1230 in thereceiver 1200) should preferably be less than about 20 degrees (e.g., 15degrees, 10 degrees, 5 degrees, and so on).

In another example, the WPT system 1000 may be designed to operate at aparticular distance between the inductor coils in the transmitter 1100and the receiver 1200. The first coil 1110/second coil 1130 and thethird coil 1210/fourth coil 1230 may be collectively tuned to increasethe energy transfer rate and efficiency. Once the inductor coils aretuned for a particular gap, subsequent changes to the gap (e.g., alarger or smaller gap) may reduce the energy transfer rate.

Operating Parameters and Electronic Properties in the WPT System

The energy transfer rate and the power efficiency of the WPT system 1000may generally depend on the electronic properties of the components inthe resonant circuits 1102 and 1202 as well as the desired operatingconditions, such as the separation gap and size constraints on the WPTsystem 1000. The energy transfer rate of the WPT system 1000 may betailored for low power and/or high power applications. In principle, theWPT system 1000 may be tailored to operate at any power level. Forexample, the energy transfer rate may vary between about 1 W to about500 kW. Generally, the power efficiency of the WPT system 1000 may varybetween about 1% to about 100%. A higher power efficiency in the WPTsystem 1000 is generally preferable for several reasons including, butnot limited to, reducing the charging time of the load 1250 (e.g., abattery), supplying more power to the load 1250 (e.g., a motor), andreducing the amount of power generated by the power source 1150.However, the desired power efficiency may also vary with theapplication. For example, a power efficiency of at least 85% (e.g., 90%,95%, or more) is preferable for charging electric vehicles.

The distance between the inductor coils of the transmitter 1100 and thereceiver 1200 may generally vary between about 0.1 cm to about 1 m. Forsome applications, the separation gap range maybe substantially smaller.For example, WPT systems 1000 used in electric vehicles may have aseparation gap ranging between about 10 mm to about 200 mm. The smallerseparation gap provides several benefits including: (1) a largermagnetic coupling coefficient k and thus a higher power efficiency, (2)less EMI in the environment due to fringe magnetic fields at the edgesof the inductor coils, and (3) better safety by preventing objects,small children, or animals from getting between the transmitter 1100 andthe receiver 1200.

The operating frequency of the WPT system 1000 may also vary betweenabout 20 kHz and about 20 MHz. However, in some applications, theoperating frequencies may be constrained to a particular band based onindustry standard guidelines. For instance, in EVs, the operatingfrequency may range between about 80 kHz and about 90 kHz (e.g., 87.5kHz) as set forth by the Society of Automotive Engineers (SAE)guidelines for wireless power transfer in light-duty plug-in/EVs.

As described above, the electronic properties of the WPT system 1000 arecodependent and may vary based on one or more operating parameters, suchas the desired energy transfer rate, power efficiency, separation gap,and operating frequency. For the transmitter 1100, the first coil 1110may have an inductance ranging between about 0.1 μH to about 100 μH. Thesecond coil 1130 may have an inductance ranging between about 0.1 μH toabout 100 μH. The first capacitor 1120 may have a capacitance rangingbetween about 0.01 μF to about 100 μF. The second capacitor 1140 mayhave a capacitance ranging between about 0.01 μF to about 100 μF.

The receiver 1200 may also exhibit similar parameter ranges. The thirdcoil 1210 may have an inductance ranging between about 0.1 μH to about100 μH. The fourth coil 1230 may have an inductance ranging betweenabout 0.1 μH to about 100 μH. The third capacitor 1220 may have acapacitance ranging between about 0.01 μF to about 100 μF. The fourthcapacitor 1240 may have a capacitance ranging between about 0.01 μF toabout 100 μF.

In some applications, the inductance of the first coil 1110, the secondcoil 1130, the third coil 1210, and the fourth coil 1230 may each betailored to be substantially larger than any stray inductance in the WPTsystem 1000. For example, the wiring used to couple the second coil 1130to the power source 1150 or the fourth coil 1230 to the load 1250 mayhave stray inductance that may affect the resonant frequency of the WPTsystem 1000. If the inductance of the second coil 1130 and/or the fourthcoil 1230 is substantially larger than the stray inductance (e.g., theinductance of the coil is 10 times larger than the stray inductance),the effects on the resonant frequency may be negligible. However, anexcessively large inductance may also not be desirable as largeinductances may result in an undesirable voltage gain, as describedbelow.

The first coil 1110, the second coil 1130, the third coil 1210, and thefourth coil 1230 may each be various types of inductors including, butnot limited to, an air cored inductor, an iron cored inductor, a ferritecored inductor, a bobbin based inductor, a toroidal inductor, a flatspiral inductor, a multilayer ceramic inductor, and any other inductorsknown to one of ordinary skill in the art. For some WPT systems 1000,the first coil 1110, the second coil 1130, the third coil 1210, and thefourth coil 1230 may have a shape, dimensions, and/or an inductance thatare substantially similar to one or more other inductor coils in the WPTsystem 1000. For some WPT systems 1000, the first coil 1110, the secondcoil 1130, the third coil 1210, and the fourth coil 1230 may each besubstantially different in terms of the shape, dimensions, and/or theinductance.

The shape and dimensions of the first coil 1110, the second coil 1130,the third coil 1210, and the fourth coil 1230 may vary so as to conformto the shape of the underlying support structure. For example, aninductor coil installed on the undercarriage of an EV may besubstantially flat. In another example, an inductor coil installed inthe bumper of an EV may be curved to conform to the bumper or vehicleframe.

For some WPT systems 1000, the operating area of the transmitter 1100and/or the receiver 1200 is determined by the shape and dimensions ofthe coils. Generally, the first coil 1110, the second coil 1130, thethird coil 1210, and the fourth coil 1230 may have a characteristicdimension (e.g., a diameter) that varies between about 100 mm to about15 m. For some applications, the operating area and, hence, the size ofthe inductor coils may be constrained. For example, the size of theinductor coils may be constrained by the size of the undercarriage in avehicle.

The first coil 1110, the second coil 1130, the third coil 1210, and thefourth coil 1230 may be formed from various electrical conductorsincluding, but not limited to, copper, aluminum, any alloys of theforegoing, or any other electrical conductors known to one of ordinaryskill in the art. Various types of wiring may also be used to form theinductor coils including, but not limited to, Litz wiring, multistrandwiring, tinsel wiring, or any other wiring known to one of ordinaryskill in the art.

The first capacitor 1120, the second capacitor 1140, the third capacitor1220, and the fourth capacitor 1240 may each be various types ofcapacitors including, but not limited to, a ceramic capacitor, a filmcapacitor, an electrolytic capacitor, a supercapacitor, or any othercapacitor known to one of ordinary skill in the art. For some WPTsystems 1000, the first capacitor 1120, the second capacitor 1140, thethird capacitor 1220, and the fourth capacitor 1240 may be formed fromtwo or more capacitors coupled in series and/or parallel in order to (1)tune the resonance frequency and/or (2) to support a higher energytransfer rate if the voltage/current rating for a single capacitor isnot sufficient.

For some WPT systems 1000, the first capacitor 1120, the secondcapacitor 1140, the third capacitor 1220, and the fourth capacitor 1240may have a shape, dimensions, and/or an inductance that aresubstantially similar to one or more other capacitors in the WPT system1000. For some WPT systems 1000, the first capacitor 1120, the secondcapacitor 1140, the third capacitor 1220, and the fourth capacitor 1240may each be substantially different in terms of the shape, dimensions,and/or the capacitance.

Tunability of the WPT System

The resonant circuits 1102 and 1202 of the WPT system 1000 may generallybe tuned to adjust various operating parameters in the WPT system 1000including, but not limited to, the resonant frequency, the voltage gain,the energy transfer rate, and the power efficiency. Tuning may beaccomplished, in part, by adjusting the inductances of the first coil1110 and/or the second coil 1130 and/or the capacitances of the firstcapacitor 1120 and/or the second capacitor 1140. For some WPT systems1000, some of these electronic parameters may be fixed during assemblyand thus cannot be changed once the WPT system 1000 is fully assembled,such as the inductances of the coils, which may be determined by thethickness and/or the number of turns of the coil. However, in some WPTsystems 1000, these electronic parameters may be tunable after assemblyof the WPT system 1000, such as the capacitances of the capacitors.

For example, the WPT system 1000 may be configured to operate over abroad range of operating frequencies, which may change during operationdepending on the application. The first capacitor 1120 and the secondcapacitor 1140 may be variable capacitors, which are capacitors withcapacitances that may be changed in a mechanical or electronic manner.The LC resonant frequency may thus be tuned by adjusting thecapacitances of each respective capacitor in the WPT system 1000 tomatch the operating frequency in order to maintain a high powerefficiency. For instance, the WPT system 1000 may include only thereceiver 1200. The receiver 1200 may receive power from othertransmitters (e.g., the conventional transmitters described above),which may operate at different frequencies. In order to maintain a highpower efficiency and a high energy transfer rate, the resonant frequencyof the receiver 1200 may be tuned to match the operating frequency ofthe transmitter. While the energy transfer rate and the power efficiencymay be lower compared to the case where the receiver 1200 is coupled tothe transmitter 1100, the receiver 1200 nevertheless may still be ableto receive and transfer power to the load 1250.

In another example, the voltage gain between the power source 1150 andthe load 1250 in the WPT system 1000 may be adjusted depending on theapplication. For example, the WPT system 1000 may provide a voltage gainranging between about 0.01 to about 100. As described above, theimpedance matching functionality in the resonant circuits 1102 and 1202is used to increase the voltage gain in the WPT system 1000 in order tocompensate the frequency splitting effect. However, in someapplications, it may not be desirable for the WPT system 1000 to operateat the highest voltage gain possible. For example, in applicationsinvolving wireless power transfer between vehicles, the WPT system 1000should preferably be a symmetric system where a vehicle receives ortransmits the same voltage and current levels with respect to anothervehicle. In other words, the voltage gain should preferably be about1.0. In another example, a vehicle may be charged using power sourcedfrom a wall socket. The socket voltage may be substantially higher thanthe car battery voltage, thus the preferred voltage gain may be lessthan 1.0.

Additional Considerations

The WPT system 1000 may also be configured to operate with various mediadisposed between the transmitter 1100 and the receiver 1200 including,but not limited to, air, water, salt water, snow, and ice. For some WPTsystems 1000, the electronic properties of the resonant circuits 1102and 1202 may be tailored to accommodate different media with differentcomplex dielectric permittivities to avoid undesirable reductions in theenergy transfer rate and the power efficiency. Some WPT systems 1000 mayinstead be configured to be insensitive to changes in the medium betweenthe transmitter 1100 and the receiver 1200 changes when the WPT system1000 is in use. For example, the separation gap between the inductorcoils of the transmitter 1100 and the receiver 1200 may be keptsufficiently small such that any drop in the magnetic field in themedium is negligible even if the medium changes during operation (e.g.,snow or ice accumulate between the transmitter 1100 and the receiver1200). The complex dielectric permittivity of media is generallyfrequency dependent. Thus, the operating frequency and the resonantfrequency may also be tuned to operate at frequencies where losses inthe medium are lower and preferably negligible. For example, iceexhibits a peak in absorption loss at a frequency of about 5-6 kHz,which decreases monotonically at lower frequencies and higherfrequencies outside the 5-6 kHz range. Thus, the operating frequency andthe resonant frequency may be tuned to frequencies with lower loss(e.g., outside the 5-6 kHz range for ice), which may also vary dependingon the gap between the transmitter 1100 and the receiver 1200, theacceptable losses in the WPT system 1000, and constraints imposed on theoperating frequency range or resonant frequency range.

The WPT system 1000 is generally not limited to the circuit componentspreviously described in the resonant circuits 1102 and 1202 shown inFIG. 3A. The resonant circuits 1102 and/or 1202 may generally includeadditional inductors and/or capacitors to further increase the energytransfer rate and/or the power efficiency of the WPT system 1000. Forsome WPT systems 1000, each additional inductor may be accompanied by acorresponding capacitor. These additional electronic components may beused to further increase the voltage gain and/or to transmit/receivemore power compared to the design shown in FIG. 3A.

As described above, the transmitter 1100 and/or the receiver 1200 may beused with other receivers and transmitters, respectively. For example,some WPT systems 1000 may include only the transmitter 1100 to be usedwith various conventional receivers. Similarly, some WPT systems 1000may include only the receiver 1200 to be used with various conventionaltransmitters. The added compatibility of the transmitter 1100 orreceiver 1200 with other systems may allow for more widespread use ofthe WPT system 1000 by utilizing transmitters and receivers alreadydeployed in the field. The increase in the energy transfer rate and thepower efficiency may not be as large compared to WPT systems 1000 thatinclude both the transmitter 1100 and the receiver 1200, but thetransmitter 1100 or the receiver 1200 may nevertheless supply or receivepower wirelessly from other systems. As described above, the transmitter1100 and the receiver 1200 may be tuned to accommodate conventionalreceivers and transmitters, respectively. For example, the resonantfrequency may be tuned to match the operating frequency of theconventional receivers and transmitters.

The transmitter 1100 and the receiver 1200 may be distinguished only bytheir connection to the power source 1150 and the load 1250. Theresonant circuits 1102 and 1202 are agnostic to their function as atransmitter or a receiver, which allows the WPT system 1000 to transmitand receive power in a bidirectional manner. For example, the resonantcircuit 1102 may be electrically coupled to a switch that togglesbetween the power source 1150 and a load. Similarly, the resonantcircuit 1202 may be electrically coupled to a switch that togglesbetween the load 1250 and a power source. In another example, the powersource 1150 and the load 1250 may both be a battery and/or asupercapacitor configured to supply power and/or receive power dependingon the operating mode. In one operating mode, the resonant circuit 1102may be coupled to the power source 1150 and the resonant circuit 1202coupled to the load 1250 such that power is transferred from theresonant circuit 1102 to the resonant circuit 1202. In another operatingmode, the resonant circuit 1102 may be coupled to a load and theresonant circuit 1202 coupled to a power source such that power istransferred from the resonant circuit 1202 to the resonant circuit 1102.In this manner, the designation of which resonant circuit corresponds tothe transmitter and the receiver is wholly dependent on the operatingmode of the WPT system 1000. The resonant circuit 1102 or 1202 in theWPT system 1000 may thus be used as either a transmitter and a receiver,reducing costs and saving space/weight.

Exemplary Designs for the WPT System

FIGS. 3B and 3C show different WPT designs. FIG. 3B shows a WPT system1002 with an alternative circuit arrangement with respect to FIG. 3Awhere the first capacitor 1120 is coupled in series between the powersource 1150 and the first coil 1110. In other words, the location of thefirst capacitor 1120 and the first coil 1110 are switched in theresonant circuit 1102. The third capacitor 1220 is also coupled inseries between the load 1250 and the third coil 1210. The WPT system1002 shown in FIG. 3B may function substantially similar to the WPTsystem 1000 shown in FIG. 3A. The WPT system 1002 shown in FIG. 3B mayalso use a tapped inductor coil instead of the first coil 1110 and thesecond coil 1130.

FIG. 3C shows a WPT system 1003 where the first coil 1110 and the secondcoil 1130 in the transmitter 1100 are replaced by a tapped inductor coil1160. The tap in the inductor coil 1160 may be coupled to the secondcapacitor 1140 as shown in FIG. 3C. Similarly, the third coil 1210 andthe fourth coil 1230 in the receiver 1200 may also be replaced by atapped inductor coil 1260 with a tap coupled to the fourth capacitor1240. Using a tapped inductor coil further simplifies the WPT system1003 by eliminating the use of separate inductor coils in the resonantcircuit 1102 and 1202.

An Exemplary Coil Assembly for a Transmitter and/or a Receiver

FIGS. 4A-4C show exemplary assemblies and designs of the first coil 1110and the second coil 1130 in the transmitter 1100. These assemblies anddesigns may also be applied to the third coil 1210 and the fourth coil1230 in the receiver 1200. As shown, the first coil 1110 and the secondcoil 1130 may each be a flat spiral coil, which is a type of air corecoil. The flat spiral coil may be comprised of one or more wires curvedto form a spiral shape with one or more turns and a center opening. Thespiral may be substantially flat. For some WPT systems 1000, the use offlat spiral coils may allow the gap to be substantially uniform betweenthe transmitter 1100 and the receiver 1200 (e.g., the transmitter 1100and the receiver 1200 are in parallel alignment). Constraining the gapin this manner may simplify tuning of other structural and electronicproperties of the resonant circuit 1102 compared to the case where thegap varies spatially across the area of the transmitter 1100 and thereceiver 1200.

As shown in FIGS. 4A-4C, the first coil 1110 and the second coil 1130may be disposed in close proximity to one another (e.g., less than 1 mm)in order to allow both inductor coils to transmit power to a receiver ata substantially similar separation gap. For example, FIG. 4A shows anexemplary assembly where the second coil 1130 is disposed in the centeropening of the first coil 1110. FIG. 4B shows another exemplary assemblywhere the first coil 1110 is disposed in the center opening of thesecond coil 1130. FIG. 4C shows yet another exemplary assembly where thesecond coil 1130 is stacked onto the first coil 1110 in a concentricmanner. For some WPT systems 1000, the first coil 1110 and the secondcoil 1130 may be arranged such that the first coil 1110 and acorresponding third coil 1210 are closer than the second coil 1130 andthe fourth coil 1230. For some WPT systems 1000, the first coil 1110 andthe third coil 1210 may be spaced farther apart compared to the secondcoil 1130 and the fourth coil 1230. In this assembly, the thickness ofthe first coil 1110 or the third coil 1130 may also affect theseparation gap depending on the arrangement between transmitter 1100 andthe receiver 1200.

The respective ends of the first coil 1110 and the second coil 1130 maybe electrically coupled in accordance to the circuit schematic shown inFIG. 3A. For instance, in FIG. 3A, the innermost end of the second coil1130 and the outermost end of the first coil 1110 may be coupled to thepower source 1150. The outermost end of the second coil 1103 and theinnermost end of the first coil 1110 may be coupled to both the firstcapacitor 1120 and the second capacitor 1140.

The inductance of a flat spiral coil may depend on several tunablestructural parameters including, but not limited to, the number ofturns, the inner diameter of the center opening, the outer diameter, thedistance between adjacent windings in the spiral, the diameter/thicknessof the wire, and the cross-sectional shape of the wiring. Additionally,the wiring used to form the flat spiral coil may include one or morestrands (e.g., a multistrand wire, a Litz wire). Various electricallyconducting materials may also be used to form the flat spiral coilincluding, but not limited to, copper, aluminum, any alloys of theforegoing, or any other electrically conducting materials known to oneof ordinary skill in the art. Although FIGS. 4A-4C show the first coil1110 and the second coil 1130 as being substantially circular, othershapes may be used including, but not limited, an ellipse, a square, arectangle, a triangle, or any other polygonal shape known to one ofordinary skill in the art.

These parameters may be adjusted in order to support low power or highpower applications at various operating ranges. For example, to supporta larger energy transfer rate, the diameter of the flat spiral coil mayincrease and the wiring may be made thicker. These parameters may alsoenable operation over larger separation gaps between the transmitter1100 and the receiver 1200. However, a larger outer diameter coil alsoresults in a larger and heavier WPT system 1000, which may not bedesirable for certain applications where space and weight savings areimportant.

An Exemplary WPT System Using Flat Spiral Coils

FIG. 5 shows an illustration of a prototype WPT system 1000 where thefirst coil 1110, the second coil 1130, the third coil 1210, and thefourth coil 1230 are all flat spiral coils. In this exemplary prototype,the first coil 1110 is disposed in the center opening of the second coil1130. Similarly, the third coil 1210 is disposed in the center openingof the fourth coil 1230. As shown, the outer diameter of the second coil1130 and the fourth coil 1230 is 215 mm. The inner diameter of the firstcoil 1110 and the third coil 1210 is 110 mm. Each flat spiral coil isformed from a Litz wire in order to reduce undesirable conduction lossescaused by the skin effect. The wiring used in the first coil 1110 andthe third coil 1210 include two wires coupled electrically in parallelto one another. The wiring used in the second coil 1130 and the fourthcoil 1230 include four wires coupled electrically in parallel. Thediameter of each individual fiber in the Litz wire is about 0.1 mm. Inother designs, the flat spiral coils may be formed onto a printedcircuit board or other types of wiring known to one of ordinary skill inthe art. The first coil 1110 and the second coil 1130 are separated fromthe third coil 1210 and the fourth coil 1230 by a separation gap, Z.

As shown, the first coil 1110 and the second coil 1130 are identical tothe third coil 1210 and the fourth coil 1230. Thus, in principle, eitherthe first coil 1110 and the second coil 1130 or the third coil 1210 andthe fourth coil 1230 may be used as the transmitter and the receiver.The first coil 1110 and the second coil 1130 are coupled to the firstcapacitor 1120 (not shown), the second capacitor 1140 (not shown), andthe power source 1150 (not shown) based on the circuit models shown inFIG. 3A or 3B. Similarly, the third coil 1210 and the fourth coil 1230are coupled to the third capacitor 1220 (not shown), the fourthcapacitor 1240 (not shown), and the load 1250 (not shown) as shown inFIG. 3A.

For this exemplary design, the inductances and capacitances were tunedfor a separation gap of 50 mm and a resonant frequency of 87 kHz. Theinductances of the first coil 1110 and the second coil 1130 are 10 μHand 2 μH, respectively. The capacitances of the first capacitor 1120 andthe second capacitor 1140 are 0.067 μF and 1 μF, respectively.Similarly, the inductances of the third coil 1210 and the fourth coil1230 are 10 μH and 2 μH, respectively. The capacitances of the thirdcapacitor 1220 and the fourth capacitor 1240 are 0.067 μF and 1 μF,respectively. The inductances are fixed in this design, but thecapacitances may be varied, thus allowing tunability of the resonantfrequency.

FIGS. 6A and 6B show the voltage gain and the power efficiency,respectively, as a function of the operating frequency for the WPTsystem 1000 described above. As shown in FIG. 6A, the voltage gain islarger than 1 at the resonant frequency of 87 kHz. The 3 dB bandwidth ofthe voltage gain is about 31 kHz. This is substantially larger than theconventional WPT system 100 with a magnetic coupling coefficient k=0.05,which has a 3 dB bandwidth of about 5 kHz as shown in FIG. 1B.Additionally, FIG. 6B shows the power efficiency is greater than 95% atthe resonant frequency and across the 31 kHz 3 dB bandwidth.

Additional experiments were performed on the prototype WPT system 1000to evaluate the sensitivity of the power efficiency as a function of thealignment and the separation gap between the transmitter 1100 and thereceiver 1200. In these experiments, the first coil 1110 and the secondcoil 1130 in the transmitter 1100 were excited by a full-bridge powerinverter operating at 87 kHz. The AC voltage induced at the third coil1210 and the fourth coil 1230 were rectified by a synchronous rectifier.The magnetic fields generated by the first coil 1110 and the second coil1130 in the transmitter 1100 and the third coil 1210 and the fourth coil1230 in the receiver 1200 were shielded by ferrite and metal plates.

FIGS. 7A and 7B show the power efficiency as a function of a coilmisalignment, X, and the separation gap, Z, respectively. The coilmisalignment is the lateral distance between the centers of thetransmitter 1100 and the receiver 1200 (the lateral deviation fromconcentric alignment). As shown in FIG. 7A, the power efficiency remainsgreater than 91% for a coil misalignment, X, less than about 40 mm. Asshown in FIG. 7B, the power efficiency exhibits a peak centered aboutthe preferred separation gap of 50 mm. As described above, theinductances and capacitances of the WPT system 1000 may be tailored fora specific separation gap. When the separation gap deviates from thepreferred separation gap, suboptimal inductive coupling may occur thusreducing the power efficiency. Nevertheless, the power efficiency inFIG. 7B remains greater than 91% for a separation gap, Z, between 40 mmand 70 mm. Therefore, this data shows that the prototype WPT system 1000can achieve a high power efficiency even when the inductor coils in thetransmitter 1100 and the receiver 1200 are misaligned and when theseparation gap deviates from the preferred separation gap. Furthermore,the prototype WPT system 1000 does include tuning capacitors andswitches rated for high power applications, which substantially reducethe size, weight, and cost of the WPT system 1000.

CONCLUSION

All parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and the actual parameters, dimensions,materials, and/or configurations will depend upon the specificapplication or applications for which the inventive teachings is/areused. It is to be understood that the foregoing embodiments arepresented primarily by way of example and that, within the scope of theappended claims and equivalents thereto, inventive embodiments may bepracticed otherwise than as specifically described and claimed.Inventive embodiments of the present disclosure are directed to eachindividual feature, system, article, material, kit, and/or methoddescribed herein. In addition, any combination of two or more suchfeatures, systems, articles, materials, kits, and/or methods, if suchfeatures, systems, articles, materials, kits, and/or methods are notmutually inconsistent, is included within the inventive scope of thepresent disclosure.

Also, various inventive concepts may be embodied as one or more methods,of which at least one example has been provided. The acts performed aspart of the method may in some instances be ordered in different ways.Accordingly, in some inventive implementations, respective acts of agiven method may be performed in an order different than specificallyillustrated, which may include performing some acts simultaneously (evenif such acts are shown as sequential acts in illustrative embodiments).

All publications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

1. A wireless power receiver, comprising: a resonant circuit to receivepower from a wireless power transmitter via wireless magnetic resonancecharging at a voltage gain of about 1 and an efficiency of at least 95%at a resonant frequency between about 80 kHz and about 90 kHz, theresonant circuit comprising: a first coil; a second coil coupled inseries and concentric with the first coil and stacked on the first coil,the first coil and the second coil receiving the power from a third coiland a fourth coil in the wireless power transmitter at the resonantfrequency during operation of the wireless power receiver; a firstcapacitor coupled in series with the first coil; and a second capacitorcoupled in parallel with the first coil and the first capacitor,wherein: the first coil is magnetically coupled to the second coil; thefirst coil and the second coil are operable to, when physically alignedwith the wireless power transmitter, be magnetically coupled to thethird coil and the fourth coil; and the first capacitor and the secondcapacitor do not act as an open circuit during operation of the wirelesspower receiver.